Is your horse's back "uphill"? Meaning the withers are a bit higher than the topline? My paint is a "downhill" horse (high croup) and I had the reverse of your issue. Even saddles that were custom made to fit to a tee, with a measuring guage, slid forward and gouged him in the shoulder. It's gravity, not fit. When I add my weight and some motion, it would slide downhill. In your case that would be back (if his back slopes that way). My grief was finally solved (and discovered the issue) by Colin Dangaard of Australian Stock Saddle Company in Malibu. He built up a "lift" of one inch on the tree in my new saddle that he made. In my case the riser was in the front half of my tree. In an uphill horse, it would be the back half of the tree. This makes the top of the saddle (the part you sit in) level, even though the back has a slope. Does this make sense to you? :)
